How to prevent ComImport interface function reordering?

Question

Hi!

I have strange problem while writing c# ComImport interface. Same name functions are reordered!?

Example....

In C++ IDCompositionVisual function order is:

STDMETHOD(SetOffsetX)(THIS_ float offsetX ) PURE
STDMETHOD(SetOffsetX)(THIS_ In IDCompositionAnimation* animation ) PURE
STDMETHOD(SetOffsetY)(THIS_ float offsetY ) PURE
STDMETHOD(SetOffsetY)(THIS_ In IDCompositionAnimation* animation ) PURE

and, if I set c# ComImport order like in C++, like this...

[PreserveSig] int SetOffsetX([In] Single offsetX);
[PreserveSig] int SetOffsetX([In] IDCompositionAnimation animation):
[PreserveSig] int SetOffsetY([In] Single offsetY);
[PreserveSig] int SetOffsetY([In] IDCompositionAnimation animation);

I get error calls in C#, crash!!!

When I have reordered same names functions like this...

[PreserveSig] int SetOffsetX([In] IDCompositionAnimation animation):
[PreserveSig] int SetOffsetX([In] Single offsetX);
[PreserveSig] int SetOffsetY([In] IDCompositionAnimation animation);
[PreserveSig] int SetOffsetY([In] Single offsetY);

then all calls are working!!!

How to prevent this from happening?
(This is happening for all same named functions)

Thanks in advance!
Vladimir

Answer 1

COM does not support overloaded methods in an interface. If you tried to declare an overloaded method in the IDL for a COM object the MIDL compiler would complain about the redefinition and fail.

There is a discussion about how the vtable pointers for overloaded C++ methods are reversed here - inversion-of-generated-vtable-functions-order-for-functions-with-the-same-name